The present invention relates generally to fabricating planar waveguide structures, and particularly to fabricating silicon germanium (SiGe) waveguide structures.
The advent of the information age has seen an increasing drive toward extremely high-speed applications, leading to an increasing use of optical circuits in communication systems. Planar optical waveguides are widely used as components in optical communication systems. A typical planar waveguide structure comprises a lower cladding region, a light guiding core region and an upper cladding region. The light guiding core region has a higher index of refraction than either the lower or the upper cladding regions.
SiGe waveguides are of particular interest because both optical and electronic devices can be integrated on a single silicon substrate. Silicon is a well established substrate for electronic circuits, and high quality silicon is readily available at low cost. Additionally, both Si and SiGe are transparent in the 1300 nm and 1500 nm telecommunications wavelengths and further, SiGe has a higher index of refraction than Si. Finally, the optical and electrical properties of SiGe waveguides can be adjusted by varying the Ge concentration.
Although, SiGe waveguide structures are desirable for their electrical and optical properties, several problems exist with fabricating low loss SiGe waveguides. Generally, a SiGe waveguide structure comprises a lower cladding primarily composed of silicon, a silicon germanium core and an upper cladding also composed primarily of silicon. The lattice constant of Ge is 4% larger than that of Si. Thus, when SiGe is grown on pure silicon, this difference in lattice constants may cause very high misfit and threading dislocation densities in the structure. These dislocations may lead to increased optical losses in the waveguide structure.
Moreover, in a typical SiGe waveguide structure, the distribution of Ge in the vertical direction is different from the distribution of Ge in the horizontal direction. This asymmetry leads to birefringence effects. Generally, a light wave traveling down a waveguide comprises two orthogonally polarized modes, one perpendicular to the substrate and the other parallel to the substrate. In conventional SiGe waveguide structures, the perpendicular mode sees a different index of refraction from the parallel mode, leading to a dispersion of the transmitted signal.
Additionally, waveguide structures used for optical telecommunications typically require core thicknesses in the range of 2-10 xcexcm. Current methods of producing SiGe waveguide structures are very slow and impractical for growing SiGe waveguide structures of such thicknesses. Further, such slow growth processes may increase the amount of contaminants in the waveguide structure. These contaminants may also contribute to optical losses in the waveguide structure.
Therefore, there is a need in the art for a low loss SiGe waveguide fabrication process that is suitable for large scale production of SiGe waveguide structures.
There are several embodiments of the invention.
In one embodiment of the invention, a method of forming a planar waveguide structure comprises forming a first graded layer on a substrate, wherein the first graded layer comprises a first and a second optical material, wherein the concentration of the first optical material increases with the height of the first graded layer; and forming a second graded layer on the first graded layer, the second graded layer comprising the first and second optical materials wherein the concentration of the first optical material decreases with the height of the second graded layer. The method may also include forming a blocking layer between the substrate and the first graded layer and forming an upper cladding layer over the second graded layer.
In one embodiment, the method may also include forming a uniform layer between the first and second graded layers, the uniform layer containing first and second optical materials wherein the concentration of the first optical material is constant. In another embodiment, the uniform layer is formed directly over the substrate and a cladding layer may be formed directly over the uniform layer. This embodiment may also include a blocking layer between the substrate and the uniform layer. In yet another embodiment, a uniform layer is formed directly over the substrate, a graded layer is formed immediately over the uniform layer and a cladding layer is optionally formed over the graded layer.
In one embodiment, the first optical material is germanium and the second optical material is silicon. In one embodiment, the blocking layer, the first and second graded layers, the uniform layer and/or the cladding layer are formed epitaxially.
Optionally, the method also includes etching a pattern in the substrate and then forming the blocking layer, graded layers and uniform layer, if used, so as to conform to the shape of the pattern.
In another embodiment, the etching step is performed after the first graded layer and the uniform layer are formed. In this embodiment, the pattern is etched into the uniform layer and the first graded layer and the second graded layer is then formed over the patterned etched layers. In embodiments that do not contain the uniform layer, the pattern is etched into the first graded layer and then the second graded layer is formed over the etched patterned first graded layer. In embodiments that do not contain the first or second graded layers, the pattern is etched in the uniform layer, and an upper cladding layer is optionally formed on the etched patterned uniform layer.